1. Field of the Invention
This invention relates to methods of manufacturing polymeric medical devices, in particular, stents.
2. Description of the State of the Art
This invention relates to manufacturing of biodegradable and bioresorbable medical devices. These devices include, but are not limited to radially expandable endoprostheses, that are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices that function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of a bodily passage or orifice. In such treatments, stents dilate stenotic regions, hold dissections in place, and prevent vasospasm and abrupt closure following angioplasty in the vascular system. A complication following stenting or balloon angioplasty is restenosis. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
Stents are typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts, formed from wires, tubes, or sheets of material rolled into a cylindrical shape. This scaffolding gets its name because it physically holds open and, if desired, expands the wall of the passageway. Typically, stents are capable of being compressed or crimped onto a catheter so that they can be delivered to and deployed at a treatment site.
Delivery includes inserting the stent through small lumens using a catheter and transporting it to the treatment site. Deployment includes expanding the stent to a larger diameter once it is at the desired location. Mechanical intervention with stents has reduced the rate of restenosis as compared to balloon angioplasty.
Stents are used not only for mechanical intervention but also as vehicles for providing biological therapy. Medicated stents provides biological therapy through local administration of a therapeutic substance. A medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier that includes an active or bioactive agent or drug. A polymeric scaffolding may also serve as a carrier of an active agent or drug.
A biodegradable stent must be able to satisfy a number of mechanical requirements. The stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel. Therefore, a stent must possess adequate radial strength. Radial strength, which is the ability of a stent to resist radial compressive forces, relates to a stent's radial yield strength and radial stiffness around a circumferential direction of the stent. A stent's “radial yield strength” or “radial strength” (for purposes of this application) may be understood as the compressive loading, which if exceeded, creates a yield stress condition resulting in the stent diameter not returning to its unloaded diameter, i.e., there is irrecoverable deformation of the stent. When the radial yield strength is exceeded the stent is expected to yield more severely and only a minimal force is required to cause major deformation.
A biodegradable stent may be designed to fulfill it clinical purpose and then be resorbed. Once expanded, such a stent should adequately maintain its size and shape for a period of time to maintain patency or provide structural tissue support of a blood vessel despite the various forces that may come to bear on it, including the cyclic loading induced by the beating heart. In an exemplary treatment, a stent provides patency to a lumen for a period of time, its mechanical properties decline, it loses structural integrity, and then it is resorbed.
However, there are several challenges in making a bioabsorbable polymeric stent that provides desirable treatment outcomes. The mechanical and degradation behavior of a biodegradable stent, and the potential clinical outcomes, are quite sensitive to the properties of the biodegradable polymer of a finished product. These challenges include developing manufacturing methods that provide properties of the finished product that provide the desirable treatment outcomes.